Many of older vehicle brake systems are hydraulic brake systems, which include a brake disk and friction pads driven by hydraulic cylinders and pressed against the brake disk. But with the recent introduction of new brake control systems such as anti-lock brake systems (ABS), electric brake systems, which do not use hydraulic circuits, are gathering much attention.
Electric brake systems include a rotary shaft driven by an electric motor, and a linear motion mechanism including a linear motion member and configured to convert the rotation of the rotary shaft to linear motion of the linear motion member such that the friction pad is pressed against the brake disk by the linear motion member. Many of such electric brake systems include a sensor for detecting the pressing force with which the friction pad is pressed against the brake disk in order to control the braking force to a desired value.
Electric brake systems including a sensor for detecting the pressing force of the friction pad are disclosed e.g. in JP Patent Publication 2003-287063A, JP Patent Publication 2003-014018A, and JP Patent Publication 2004-204990A.
The electric brake system disclosed in JP Patent Publication 2003-287063A has a caliper body including opposed pieces facing each other on both sides of the brake disk, and a bridge through which the opposed pieces are coupled together. A linear motion mechanism is mounted in the caliper body such that the friction pad is pressed against the brake disk by the linear motion member of the linear motion mechanism. A strain gauge is mounted in the caliper body at its portion where the bridge is joined to one of the opposed pieces to detect the pressing force of the friction pad.
The electric brake system disclosed in JP Patent Publication 2003-014018A also includes a caliper body including opposed pieces facing each other on both sides of the brake disk and coupled together by a bridge, and a linear motion mechanism mounted in the caliper body. A pair of electrodes are embedded in the linear motion member of the linear motion mechanism to detect the pressing force of the friction pad by measuring the electrical resistance between the electrodes, which changes corresponding to strain.
The electric brake system disclosed in JP Patent Publication 2004-204990A also includes a caliper body including opposed pieces facing each other on both sides of the brake disk and coupled together by a bridge, and a linear motion mechanism mounted in the caliper body. The pressing force of the friction pad is detected by measuring the pressure in a hydraulic pressure chamber defined in the linear motion member of the linear motion mechanism, which changes corresponding to strain.
Due to frictional heat generated between the friction pad and the brake disk, the temperature of and around the friction pad tends to rise.
In the electric brake system disclosed in JP Patent Publication 2003-287063A, since the strain sensor for detecting the pressing force of the friction pad is provided at the portion of the caliper body where the bridge is joined to one of the opposed pieces, the strain sensor tends to be heated to a high temperature, which makes it impossible to provide a circuit for processing the sensor signal sufficiently close to the strain sensor. This increases noise in the sensor signal. Also, when the caliper body is heated to high temperature, the temperature distribution become uneven, thus generating thermal strain. Since the strain sensor picks up such thermal strain too, it is difficult to measure only the strain due to the pressing force of the friction pad with high accuracy.
In the electric brake system of JP Patent Publication 2003-014018A too, since the electrodes for detecting the pressing force of the friction pad are provided in the linear motion member, the electrodes tend to be heated to high temperature, so that it is impossible to provide the signal processing circuit sufficiently close to the electrodes, so that noise in the sensor signal increases. Also, since the linear motion member, in which the electrodes are embedded, moves, it is necessary to arrange wires leading to the electrodes so as to be movable. This increases costs.
In the electric brake system disclosed in JP Patent Publication 2004-204990A, if hydraulic fluid in the hydraulic chamber leaks and air enters the hydraulic pressure chamber, the output of the hydraulic pressure sensor does not correspond to the pressing force of the friction pad any more. Thus this system has a problem in long-term reliability. In order to maintain liquid tightness of the hydraulic pressure chamber with high reliability, high costs are required.